At THIS POINT you have a datetime object and can pull any of the details from that you need, for example for the datetime name, instead of this:

//Add the 'EVENTDATE' merge var and use the datetimes start_date for the value.
$merge_vars['EVENTDATE'] = $datetime->start_date( 'Y-m-d' );

You could use:

$merge_vars['TICKETTYPE'] = $datetime->name();

Which sets a merge var call TICKETTYPE to the datetime name.

However, just to clarify, with the above you don’t actually have different ‘ticket types’ you have tickets assigned to datetimes (which you are using as a ticket type/group) so what you are actually sending over is just the datetime name.